Electric motor

ABSTRACT

It comprises stator including stator core having yoke and a plurality of teeth protruded from yoke, which is formed with slots between adjacent teeth, and rotor having rotor core and permanent magnet formed with a plurality of magnetic poles, which confronts tip ends of teeth via gaps, wherein rotor core is formed by rotor core materials circumferentially equally divided into the predetermined number of divisions, and the least common multiple being N for the number of slots and the number of magnetic poles and the least common multiple being M for the number of slots and the number of divisions, then N is equal to M.

This Application is a U.S. National Phase Application of PCTInternational Application PCT/JP2008/000857.

TECHNICAL FIELD

The present invention relates to an electric motor using a rotor coreformed by a plurality of circumferentially divided rotor core materials.

BACKGROUND ART

As to a rotor core formed by a plurality of circumferentially dividedrotor core materials, technology as mentioned in Patent document 1 iscommonly known. The technology mentioned in Patent document 1 is suchthat these rotor core materials are connected to each other with someplay provided via junctions in such manner as to be positioned in anannular fashion. And, in this condition, they are set in a mold andintegrated into one piece by a steel frame and resin in the mold.

Due to the technology mentioned in Patent document 1, when rotor corematerials are set in a mold, rotor core materials are connected to eachother with some play provided via junctions, and therefore, the rotorcore materials can be set in the mold while providing clearance betweenthe mold and the rotor core. In this way, it is possible to improve theworkability. Also, when resin is used for the purpose of molding, therotor core is pressed in the direction of inner diameter due to theresin molding pressure, causing the play between rotor core materials tobe removed, and it is possible to secure the dimensional accuracy.

However, when a rotor core formed by rotor core materials as mentionedin Patent document 1 is used, gaps are generated between rotor corematerials. As a result, abrupt change of the magnetic flux is generatedbetween the rotor core material and the gap. This gives rise to theproblem of increase of cogging torque and torque ripple.

Patent document 1 Unexamined Japanese Patent Publication 2006-187176

SUMMARY OF THE INVENTION

The electric motor of the present invention has a configuration asdescribed in the following.

That is, it comprises a stator including a stator core having a yoke anda plurality of teeth protruded from the yoke, which is formed with slotsbetween the teeth adjacent to each other and winding wound around thestator core, and a rotor rotatably held against the stator, having arotor core and a plurality of magnetic poles held by the rotor core andconfronting tip ends of the teeth via gaps. The rotor core is formed byrotor core materials circumferentially equally divided into apredetermined number of divisions. And, the least common multiple beingN for the number of slots and the number of magnetic poles, and theleast common multiple being M for the number of slots and the number ofdivisions, then N is equal to M in the configuration.

In this configuration, the characteristic of cogging torque obtained issame as in the case of using a rotor core integrally formed.Accordingly, it is possible to reduce cogging torque and torque ripplewhile using a rotor core formed by a plurality of rotor core materials.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view of an electric motor in the preferredembodiment 1 of the present invention.

FIG. 2 is a partially enlarged view of one rotor core material of therotor core of FIG. 1.

FIG. 3 shows the relationship between the rotor rotating position andcogging torque of the electric motor in the preferred embodiment 1.

FIG. 4 shows the relationship between the rotor rotating position andcogging torque with the rotor core of the electric motor integrallyformed, where the number of slots S is 18 and the number of poles P is30.

FIG. 5A shows the relationship between rotor rotating position andcogging torque where N≠M (number of slots S=18, number of poles P=30,number of divisions D=2).

FIG. 5B shows the relationship between rotor rotating position andcogging torque where N≠M (number of slots S=18, number of poles P=30,number of divisions D=3).

FIG. 5C shows the relationship between rotor rotating position andcogging torque where N≠M (number of slots S=18, number of poles P=30,number of divisions D=6).

FIG. 6A shows the relationship between rotor rotating position andcogging torque where N=M (number of slots S=18, number of poles P=30,number of divisions D=10).

FIG. 6B shows the relationship between rotor rotating position andcogging torque where N=M (number of slots S=18, number of poles P=30,number of divisions D=15).

FIG. 6C shows the relationship between rotor rotating position andcogging torque where N=M (number of slots S=18, number of poles P=30,number of divisions D=30).

FIG. 7 shows the relationship between rotor rotating position andcogging torque with the rotor core of the electric motor integrallyformed, where the number of slots S is 12, and the number of poles P is20.

FIG. 8A shows the relationship between rotor rotating position andcogging torque where N≠M (number of slots S=12, number of poles P=20,number of divisions D=2).

FIG. 8B shows the relationship between rotor rotating position andcogging torque where N≠M (number of slots S=12, number of poles P=20,number of divisions D=4).

FIG. 9A shows the relationship between rotor rotating position andcogging torque where N=M (number of slots S=12, number of poles P=20,number of divisions D=5).

FIG. 9B shows the relationship between rotor rotating position andcogging torque where N=M (number of slots S=12, number of poles P=20,number of divisions D=10).

FIG. 9C shows the relationship between rotor rotating position andcogging torque where N=M (number of slots S=12, number of poles P=20,number of divisions D=20).

FIG. 10 is a sectional view of an electric motor in the preferredembodiment 2 of the present invention.

FIG. 11 shows the relationship between outer rotor rotating position andcogging torque with the outer rotor core integrally formed.

FIG. 12 shows the relationship between outer rotor rotating position andcogging torque where Nout=Mout (number of outer slots So=18, number ofouter rotor poles Po=30, number of outer rotor core divisions Do=5).

FIG. 13 shows the relationship between inner rotor rotating position andcogging torque with the inner rotor core integrally formed.

FIG. 14 shows the relationship between inner rotor rotating position andcogging torque where Nin=Min (number of inner slots Si=18, number ofinner rotor poles Pi=30, number of inner rotor core divisions Di=5).

DESCRIPTION OF REFERENCE MARKS

-   1, 100 Rotary shaft-   11, 111 Stator-   12, 112 Yoke-   13 Teeth-   14, 114 Stator core-   15 Slot-   21 Rotor-   22, 222, 232 Magnet burying hole-   23 Rotor core-   25, 225, 235 Permanent magnet-   26 Rotor core material-   113 Outer teeth-   115 Outer slot-   123 Inner teeth-   125 Inner slot-   221 Outer rotor-   223 Outer rotor core-   226 Outer rotor core material-   231 Inner rotor-   233 Inner rotor core-   236 Inner rotor core material

PREFERRED EMBODIMENTS FOR CARRYING OUT THE INVENTION

The preferred embodiments of the present invention will be described inthe following with reference to the drawings.

Preferred Embodiment 1

FIG. 1 is a sectional view of an electric motor in the preferredembodiment 1 of the present invention.

The electric motor in the preferred embodiment 1 comprises stator 11 androtor 21.

Stator 11 includes stator core 14 formed for example by punching by apress and laminating a plurality of thin steel sheets having highmagnetic permeability, and winding (not shown) wound around stator core14. Stator core 14 has nearly annular yoke 12 and a plurality of teeth13 protruding outward from yoke 12, and slot 15 is formed between teeth13 adjacent to each other. The winding is wound around stator core 14 ina concentrated fashion and housed in slot 15.

Rotor 21 includes rotor core 23 formed with a plurality of magnetburying holes 22, and permanent magnets 25 to be buried in respectivemagnet burying holes 22. Permanent magnets 25 are disposed for formingmagnetic poles at rotor 21, and such permanent magnets 25 are kept andhoused in respective magnet burying holes 22.

Rotor 21 having such a configuration is arranged outside the stator 11and confronts the tip ends of teeth 13 of stator 11 via gaps. Rotor 21is fixed via a bottomed cylindrical rotor cup (not shown) fixed on thebottom against rotary shaft 1 and rotatably journaled on stator 11.

And, rotor core 23 is formed by a plurality of rotor core materials 26.Rotor core materials 26 have such a structure that rotor core 23 iscircumferentially equally divided into a predetermined number ofdivisions. Rotor core 23 is formed by combining such rotor corematerials 26 for the number of divisions.

Rotor core material 26 is formed with a convex-concave shape at aportion where it is combined with adjacent rotor core material 26. And,adjacent rotor core materials 26 are secured with their convex andconcave shapes engaged with each other. The combination of adjacentrotor core materials 26 is also allowable to be secured by usingadhesive instead of engaging the convex shape with the concave shape.

FIG. 2 is a partially enlarged view of one rotor core material 26 ofrotor core 23 of FIG. 1.

Rotor core 23 in the preferred embodiment 1 is formed by rotor corematerials 26 circumferentially divided into 5 divisions as thepredetermined number of divisions so that they are spaced apart at equalintervals. In the present preferred embodiment 1, the number of slots Sof stator 11 is 18, and the number of poles P of rotor 21 is set to 30.Also, rotor core 23 is formed by five rotor core materials 26, andtherefore, the number of divisions D is 5. Accordingly, the least commonmultiple being N for the number of slots S and the number of magneticpoles P, then the number of slots S is 18 and the number of poles P is30. Therefore, the least common multiple N is 90. Also, the least commonmultiple being M for the number of slots S and the number of divisionsD, then the number of slots S is 18 and the number of divisions D is 5.Therefore, the least common multiple M is also 90. That is, in thepresent preferred embodiment 1, N=M (N is equal to M).

The reason for N=M in the present preferred embodiment 1 is explained inthe following. Regarding the relationship between rotor rotatingposition and cogging torque, the description is given in the followingby using an example of comparison, changing the number of slots S ofrotor core, the number of magnetic poles S, and the number of divisionsD.

First, FIG. 3 shows the relationship between rotor rotating position andcogging torque of the electric motor in the preferred embodiment 1. Thatis, FIG. 3 shows the relationship between rotor rotating position andcogging torque where the number of slots S is 18, the number of poles Pis 30, and the number of divisions D is 5. As shown in FIG. 3, in thecase of the electric motor in the preferred embodiment 1, the maximumvalue and minimum value of cogging torque are nearly identical with eachother every cycle of cogging torque. And, the cogging torque isgenerated in a range of about −0.1 N·m to 0.1 N·m.

Next, FIG. 4 shows the relationship between rotor rotating position andcogging torque with the rotor core of the electric motor integrallyformed where the number of slots S is 18 and the number of poles S is30. That is, in the electric motor having characteristics shown in FIG.4, the rotor core is integrally formed, and the number of rotor corematerials is 1, and the number of divisions D is 1. In this case, N isnot equal to M, but the rotor core is integrally formed, therefore asshown in FIG. 4, the cogging torque is reduced to a value ranging from−0.1 N·m to 0.1 N·m.

Described next is such a case that the rotor core is divided into aplurality of portions and N is not equal to M.

FIG. 5A shows the relationship between rotor rotating position andcogging torque where N≠M (number of slots S=18, number of poles P=30,number of divisions D=2). FIG. 5B shows the relationship between rotorrotating position and cogging torque where N≠M (number of slots S=18,number of poles P=30, number of divisions D=3). FIG. 5C shows therelationship between rotor rotating position and cogging torque whereN≠M (number of slots S=18, number of poles P=30, number of divisionsD=6).

As shown in FIG. 5A to FIG. 5C, when N≠M, the cogging torque varies inmaximum value and minimum value every cycle of cogging torque. And, itgives rise to the occurrence of such rotor rotating position that thecogging torque exceeds the range of −0.1 N·m to 0.1 N·m.

Described next is such a case that the rotor core is divided into aplurality of portions the same as in the preferred embodiment 1, and Nis equal to M.

FIG. 6A shows the relationship between rotor rotating position andcogging torque where N=M (number of slots S=18, number of poles P=30,number of divisions D=10). FIG. 6B shows the relationship between rotorrotating position and cogging torque where N=M (number of slots S=18,number of poles P=30, number of divisions D=15). FIG. 6C shows therelationship between rotor rotating position and cogging torque whereN=M (number of slots S=18, number of poles P=30, number of divisionsD=30).

As shown in FIG. 6A to FIG. 6C, when N=M, as compared with the case ofN≠M, the characteristic of cogging torque generation is similar to thegeneration of cogging torque with the rotor core integrally formed asshown in FIG. 4. That is, when N=M, even in case of changing the numberof divisions D, the maximum value and minimum value of cogging torqueare nearly identical with each other every cycle of cogging torque. And,cogging torque is generated in a range of −0.1 N·m to 0.1 N·m. That is,when N=M, cogging torque is generated in a range of −0.1 N·m to 0.1 N·meven with the rotor rotating position varied.

Described next is such a case that the number of slots S and the numberof poles P are changed.

First, FIG. 7 shows the relationship between rotor rotating position andcogging torque with the rotor core of the electric motor integrallyformed where the number of slots S is 12 and the number of poles P is20. That is, in the electric motor having characteristics shown in FIG.7, the rotor core is integrally formed, therefore the number of rotorcore materials is 1, and the number of divisions D is 1. In this case, Nis not equal to M, but the rotor core is integrally formed, therefore asshown in FIG. 7, the cogging torque is reduced to a value ranging from−0.02 N·m to 0.02 N·m.

Described next is such a case that the rotor core is divided into aplurality of portions and N is not equal to M.

FIG. 8A shows the relationship between rotor rotating position andcogging torque where N≠M (number of slots S=12, number of poles P=20,number of divisions D=2). FIG. 8B shows the relationship between rotorrotating position and cogging torque where N≠M (number of slots S=12,number of poles P=20, number of divisions D=4).

As shown in FIG. 8A and FIG. 8B, when N≠M, even with the number of slotsS and the number of poles P changed, the cogging torque varies inmaximum value and minimum value every cycle of cogging torque. And, itgives rise to the occurrence of such a rotor rotating position that thecogging torque exceeds the range of −0.02 N·m to 0.02 N·m.

Described next is such a case that the number of slots S and the numberof poles P are equal to those with the rotor core integrally formed, therotor core is divided into a plurality of portions, and N is equal to M.

FIG. 9A shows the relationship between rotor rotating position andcogging torque where N=M (number of slots S=12, number of poles P=20,number of divisions D=5). FIG. 9B shows the relationship between rotorrotating position and cogging torque where N=M (number of slots S=12,number of poles P=20, number of divisions D=10). FIG. 9C shows therelationship between rotor rotating position and cogging torque whereN=M (number of slots S=12, number of poles P=20, number ofdivisions=20).

As shown in FIG. 9A to FIG. 9C, when N=M, as compared with the case ofN≠M, the characteristic of cogging torque generation is similar to thegeneration of cogging torque with the rotor core integrally formed asshown in FIG. 7. That is, when N=M, even in case of changing the numberof divisions D, the maximum value and minimum value of cogging torqueare nearly identical with each other every cycle of cogging torque. And,cogging torque is generated in a range of −0.02 N·m to 0.02 N·m.

As described above, the electric motor in the preferred embodiment 1comprises rotor core 23 formed by rotor core materials 26circumferentially equally divided into a predetermined number ofdivisions, wherein the least common multiple N for the number of slots Sand the number of magnetic poles P is equal to the least common multipleM for the number of slots S and the number of divisions D. Therefore,even in case of using rotor core 23 formed by a plurality of rotor corematerials 26, it is possible to obtain same cogging torquecharacteristics as in the case of using a rotor core integrally formed.Accordingly, according to the present invention, cogging torque can bereduced in an electric motor having a rotor core formed by a pluralityof rotor core materials, and also, it is possible to reduce torqueripple related with cogging torque.

In the description of the preferred embodiment 1, a stator whose numberof slots S is 18 and a rotor whose number of poles P is 30 are used forthe sake of convenience in the description, but as described above, itis not limited to this combination, and when N=M, it is allowable toemploy whichever combinations of the number of slots S, the number ofmagnetic poles P, and the number of divisions D that is the number ofrotor core materials.

Also, in the preferred embodiment 1, an example of configuration withrotor 21 arranged outside the stator 11 is mentioned in the description,but it is also allowable to arrange rotor 21 inside the stator 11. Thatis, it is allowable to be configured in that the stator core has aplurality of teeth protruding inward of the yoke, and the rotor insidethe stator confronts tip ends of the teeth via gaps. This is becausecogging torque is related with air gap permeance and magnetomotive forcebetween stator and rotor, and it can be considered that cogging torqueis generated in a similar pattern when the number of slots S, the numberof magnetic poles P, and the number of divisions D are same incombination.

Preferred Embodiment 2

The preferred embodiment 2 of the present invention will be described inthe following with reference to the drawings.

FIG. 10 is a sectional view of an electric motor in the preferredembodiment 2 of the present invention.

The electric motor in the preferred embodiment 2 comprises stator 111,outer rotor 221, and inner rotor 231.

Stator 111 includes stator core 114 formed for example by punching by apress and laminating a plurality of thin steel sheets having highmagnetic permeability, and winding (not shown) wound around stator core114. Stator core 114 has nearly annular yoke 112, a plurality of outerteeth 113 protruding outward from yoke 112, and a plurality of innerteeth 123 protruded inward from yoke 112. Also, stator core 114 isformed with outer slot 115 between outer teeth 113 adjacent to eachother, and with inner slot 125 between inner teeth 123 adjacent to eachother. As to the winding system for winding around stator core 114, itis allowable to employ any one of a toroidal system, concentratedwinding system, and distributed winding system.

Outer rotor 221 includes outer rotor core 223 formed with a plurality ofmagnet burying holes 222, and permanent magnets 225 to be buried inrespective magnet burying holes 222. Inner rotor 231 includes innerrotor core 233 formed with a plurality of magnet burying holes 232, andpermanent magnet 235 to be buried in respective magnet burying holes232. Permanent magnet 225 and permanent magnet 235 are disposed for thepurpose of forming magnetic poles at each rotor.

Also, outer rotor 221 and inner rotor 231 are connected to each other.For making this connection, it is allowable to use adhesive or bolts orresin for molding. And, outer rotor 221 and inner rotor 231 areconnected to rotary shaft 100 via such adhesive or molding material.

Outer rotor 221 confronts the outer peripheral tip ends of outer teeth113 via gaps. Also, inner rotor 231 confronts the inner peripheral tipends of inner teeth 123 via gaps. And, outer rotor 221 and inner rotor231 are rotatably journaled against stator 111.

And, outer rotor core 223 and inner rotor core 233 in the preferredembodiment 2 are respectively formed by a plurality of rotor corematerials. Same as in the preferred embodiment 1, the rotor corematerials in each rotor core has a structure circumferentially equallydivided into the predetermined number of divisions. Such rotor corematerials are combined for the number of divisions to form a rotor core.

As shown in FIG. 10, outer rotor core 223 comprises outer rotor corematerials 226 circumferentially divided into five portions as thepredetermined number of divisions in such manner that they are equallyspaced apart. Inner rotor core 233 comprises inner rotor core materials236 circumferentially divided into 5 portions as the predeterminednumber of divisions in such manner that they are equally spaced apart.

In the preferred embodiment 2, the number of outer slots So of stator111 is 18, and the number of magnetic poles Po of outer rotor 221 is setto 30. Also, outer rotor core 223 is formed by five pieces of outerrotor core materials 226, and therefore, the number of divisions Do inouter rotor core 223 is 5. Accordingly, the least common multiple beingNout for the number of outer slots So and the number of outer magneticpoles Po, then the number of outer slots So is 18, and the number ofmagnetic poles Po is 30, therefore the least common multiple Nout forthese is 90. Also, the least common multiple being Mout for the numberof outer slots So and the number of divisions Do, then the number ofouter slots So is 18, and the number of divisions Do is 5, therefore theleast common multiple Mout for these is also 90. That is, in thepreferred embodiment 2, Nout=Mout (Nout is equal to Mout).

Also, in the preferred embodiment 2, the number of inner slots Si ofstator 111 is 18, and the number of magnetic poles Pi of inner rotor 231is set to 30. Also, the number of divisions Di of inner rotor core 233is 5. Accordingly, the least common multiple being Nin for the number ofinner slots Si and the number of inner magnetic poles Pi, then the leastcommon multiple Nin for these is 90. Also, the least common multiplebeing Min for the number of inner slots Si and the number of divisionsDi, then the least common multiple Min for these is also 90. That is, inthe preferred embodiment 2, Nin=Min (Nin is equal to Min).

The reason for Nout=Mout and Nin=Min in the preferred embodiment 2 isdescribed in the following. Regarding the relationship between rotorrotating position and cogging torque, the description is given in thefollowing by using an example of comparison.

First, the reason for Nout=Mout in the preferred embodiment 2 isdescribed.

FIG. 11 shows the relationship between outer rotor rotating position andcogging torque with the outer rotor core integrally formed.

Also, FIG. 12 shows the relationship between outer rotor rotatingposition and cogging torque where Nout=Mout (number of outer slotsSo=18, number of poles Po of outer rotor 221=30, number divisions Do ofouter rotor core 223=5).

The cogging torque shown in FIG. 11 and FIG. 12 is measured supposingthat cogging torque generated due to the inner rotor is zero.

As shown in FIG. 12, when Nout=Mout, the characteristic of coggingtorque generation is similar to the generation of cogging torque withthe outer rotor core integrally formed as shown in FIG. 11. That is, themaximum value and minimum value of cogging torque are nearly identicalwith each other every cycle of cogging torque. And, the cogging torqueis generated within a range of −0.1 N·m to 0.1 N·m.

The reason for Nin=Min in the preferred embodiment 2 is described in thefollowing.

FIG. 13 shows the relationship between inner rotor rotating position andcogging torque with the inner rotor core integrally formed.

Also, FIG. 14 shows the relationship between rotating position andcogging torque of inner rotor 231 where Nin=Min (number of inner slotsSi=18, number of poles Pi of inner rotor 231=30, number of divisions Diof inner rotor core 233=5).

The cogging torque shown in FIG. 13 and FIG. 14 is measured supposingthat cogging torque generated due to outer rotor is zero.

As shown in FIG. 14, when Nin=Min, the characteristic of cogging torquegeneration is similar to the generation of cogging torque with the innerrotor core integrally formed as shown in FIG. 13. That is, the maximumvalue and minimum value of cogging torque are nearly identical with eachother every cycle of cogging torque. And, the cogging torque isgenerated within a range of −0.1 N·m to 0.1 N·m.

As described above, in the electric motor of the preferred embodiment 2,outer rotor core 223 comprises outer rotor core materials 226circumferentially equally divided for the predetermined number ofdivisions Do, and inner rotor core 233 comprises inner rotor corematerials 236 circumferentially equally divided for the predeterminednumber of divisions Di. And, the least common multiple being Nout forthe number of outer slots So and the number of outer magnetic poles Po,the least common multiple being Mout for the number of outer slots Soand the number of outer rotor core divisions Do, the least commonmultiple being Nin for the number of inner slots Si and the number ofinner magnetic poles Pi, and the least common multiple being Min for thenumber of inner slots Si and the number of inner rotor core divisionsDi, then at least the setting is made so that Nout is equal to Mout, orNin is equal to Min. Therefore, even in case the rotor core arranged isformed by a plurality of rotor core materials outside and inside thestator, the characteristic of cogging torque obtained is equivalent tothat obtained with the rotor core integrally formed. Accordingly, alsodue to the preferred embodiment 2 of the present invention, in theelectric motor having a rotor core formed by a plurality of rotor corematerials, cogging torque can be reduced, and also, it is possible toreduce torque ripple related with cogging torque.

Also in the preferred embodiment 2, a stator whose number of slots So,Si is 18 and a rotor whose number of magnetic poles Po, Pi is 30 areused for the convenience of description. However, as described above, itis not limited to these combinations, and it is allowable to use anynumerical combinations with respect to the number of slots So, Si, thenumber of poles Po, Pi, and the number of divisions Do, Di that is thenumber of rotor core materials where at least Nout=Mout or Nin=Min.

INDUSTRIAL APPLICABILITY

The electric motor of the present invention is capable of reducingcogging torque and torque ripple, and it is useful as an electric motorusing a rotor core formed by circumferentially divided rotor corematerials.

The invention claimed is:
 1. An electric motor comprising: a statorincluding a stator core having a yoke and a plurality of teeth protrudedfrom the yoke, which is formed with slots between the teeth beingadjacent to each other, and winding wound around the stator core; and arotor rotatably held against the stator, having a rotor core and aplurality of magnetic poles held by the rotor core and confronting tipends of the teeth via gaps, wherein the rotor core is formed by rotorcore materials circumferentially equally divided into a predeterminednumber of divisions greater than one; and the least common multiplebeing N for the number of slots and the number of magnetic poles, andthe least common multiple being M for the number of the slots and thenumber of the divisions, then N is equal to M, wherein the stator coreincludes: a plurality of outer teeth protruded outward of the yoke, anda plurality of inner teeth protruded inward of the yoke, which is formedwith outer slots between the outer teeth being adjacent to each otherand with inner slots between the inner teeth being adjacent to eachother; the rotor comprises: an outer rotor rotatably held against thestator, having an outer rotor core and a plurality of magnetic polesheld by the outer rotor core, and confronting tip ends of the outerteeth via gaps outside the stator; an inner rotor rotatably held againstthe stator, having an inner rotor core and a plurality of magnetic polesheld by the inner rotor core, and confronting tip ends of the innerteeth via gaps inside the stator; the outer rotor core is formed byouter rotor core materials circumferentially equally divided into thepredetermined number of divisions, the inner rotor core is formed byinner rotor core materials circumferentially equally divided into thepredetermined number of divisions, and the least common multiple beingNout for the number of the outer slots and the number of the magneticpoles of the outer rotor, and the least common multiple being Mout forthe number of the outer slots and the number of divisions of the outerrotor core, the least common multiple being Nin for the number of theinner slots and the number of the magnetic poles of the inner rotor, andthe least common multiple being Min for the number of the inner slotsand the number of divisions of the inner rotor core, then at least Noutis equal to Mout, or Nin is equal to Min.